1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly to an optical pickup actuator having a lens holder of a high resonance frequency.
2. Description of the Prior Art
Recently, a data retrieving/recording system has been developed which uses an optical disc such as a laser disc or a compact disc. For retrieving or recording information from or to an optical disc, an optical pickup actuator is provided to irradiate a laser beam along a track on the optical disc and to retrieve data based on a reflected beam from the track. If a track is spirally formed on an optical disc, sectors formed on the track have different distances from the center of the optical disc due to eccentricity of the track. Accordingly, a tracking control is required to precisely irradiate a laser beam on a track in a retrieving mode. Further, a distance between an optical pickup actuator and an optical disc minutely fluctuates since the optical disc is rotated in the retrieving mode. Such distance fluctuation causes difficulty in precisely retrieving data, so that a focusing control is essential.
For the focusing control, a focusing error is detected from a laser beam reflected by an optical disk so as to produce a focus error signal. The focus error signal drives the objective lens for performing a focusing control operation. In general, the objective lens is supported and fixed by a spring on an optical head housing. A focusing actuator is operated which has a coil for the objective lens to be driven in an axial direction and in an up-and-down direction of a disc.
In the meantime, as a reproducing optical disc dealing with multimedia information, a CD-ROM and the like have been put to practical use. A CD-ROM is a medium having a diameter of 12 cm, a thickness of 1.2 mm, and a track pitch of 1.6 .mu.m, on one side of which information data of 540 MBytes can be stored. In recent years, development of a digital video disc(DVD) having video information recorded on the CD-ROM using an image compression data technology such as an MPEG (Moving Picture Experts Group) has been remarkable.
In order to carry out reproduction on a disc having a high recording density, the spot diameter of a laser beam must be diaphragmed. To do so, it is considered that the wavelength of a laser of an optical pickup device should be shortened(for example, approximately 635 nm), and that the numerical aperture(NA) of an objective lens is made large(for example, approximately 0.6). However, the aberration by inclination of a disc increases in direct proportion to the cube of the numerical aperture of the objective lens. Therefore, making the numerical aperture of the objective lens too large causes decrease of an inclination margin of the disc. Under the condition of the same numerical aperture of an objective lens, the SD specified disc having a thickness of approximately 0.6 mm which is announced as DVD has an inclination margin two times as large as that of a conventional CD having a thickness of approximately 1.2 mm, for example.
In spite of the above requirements, coexistence of a disc having a standard density and a thickness of approximately 1.2 mm(CD, CD-ROM) and a disc having a high density and a thickness of approximately 0.6 mm(DVD) is expected. Therefore, an optical pickup device which can reproduce information recorded on both the CD and the DVD is desired.
A conventional optical pickup actuator for both CD and CVD discs will be hereinafter described with reference to FIGS. 1-4.
FIG. 1 is an exploded perspective view of a conventional optical pickup actuator of an axial slide type for both the CD and the DVD disc, FIG. 2 is a view vertically cross-sectioned along a line II--II of FIG. 1, FIG. 3 is a perspective view of a magnet mounted in the conventional optical pickup actuator of an axial slide type for both the CD and the DVD of FIG. 1, and FIG. 4 is another exploded perspective view of the conventional optical pickup actuator of an axial slide type for both the CD and the DVD.
As shown in FIGS. 1-3, the conventional optical pickup actuator of an axial slide type for both the CD and the DVD is constituted with a cylindrical lens holder 110, lens containers 113 and 115, winding coils 120, a cylindrical yoke 130, a shaft 135, a first magnet pair 140, and a second magnet pair 150. The shaft 135 protrudes upwardly from the bottom of the yoke 130. The first magnet pair 140 is disposed along the inner surface of the yoke 130. The second magnet pair 150 is also disposed along the inner surface of the yoke 130 perpendicular to the first magnet pair 140.
The lens holder 110 has the winding coils 120 on the periphery thereof each of which corresponds to the first and the second magnet pair 140 and 150. The lens containers 113 and 115 each protrude from the upper surface of the lens holder 110. An objective lens 112 for the CD and an objective lens 114 for the DVD are mounted in the lens containers 113 and 115, respectively. Guiding grooves 117 are formed on the periphery of the lens holder 110 directly under the lens containers 113 and 115. The guiding grooves 117 are formed along the laser beam direction so that the guiding grooves 117 are selectively aligned with a through-hole 119 formed in the bottom of the yoke 130, through which a laser beam passes.
The lens holder 110 has an axial hole 118 into which the shaft 135 is inserted. The lens holder 110 is lifted up from the bottom of the yoke 130 by a force resulting from interaction between the first and the second magnet 140 and 150. The magnetizable material is mounted in the center portion of the winding coils 120 which are placed on the periphery of the lens holder 110.
The lens containers 113 and 115 protrude in the axial direction of the objective lens 112 and 114 and on one side of the upper surface of the lens holder 110. Accordingly, the resonance frequency of the lens holder 110 becomes low due to weight of the lens holder 110. The low resonance frequency results in a drawback in that the actuator is easily influenced by a parasitic resonance and outer disturbance with ease. The objective lens 112 and 114 are selectively aligned with the laser beam axis as the lens holder 110 is rotated. The first and second magnet pair 140 and 150 are mounted to serve the movement of the lens holder 110 while preventing the lens containers 113 and 115 from moving. The height of the yoke 130 is higher than the sum of the heights L1 and H1 in order for the lens holder 110 and the objective lens 114 to be protected, wherein reference numeral L1 denotes the height of the second magnet 150 and reference numeral H1 denotes the height from the second magnet 150 to the lens container 115. Accordingly, since the optical pickup actuator becomes larger, the size of the optical pickup actuator can not be minimized.
Further, the height of the yoke 130 is closely related to the structure of the second magnet 150 as shown in FIG. 3. The second magnet 150 is shown in FIG. 3, and the same is applied to the first magnet 140. That is, a magnet M1 made of NdFe is magnetized into N pole and S pole. Non-magnetized region m is formed between the N pole and the S pole. No magnetic field can be generated in the non-magnetized region m. Accordingly, in order to obtain a magnetic field necessary for moving the lens holder 110, the magnet M1 must be lengthened by the length of the non-magnetized region, so that the height of the yoke 130 is higher and the radius of the yoke 130 is longer. Therefore, the size of the optical pickup actuator can not be minimized.
FIG. 4 is another exploded perspective view of the conventional optical pickup actuator of an axial slide type for both the CD and the DVD. The optical pickup actuator has a cylinder 210 on which focusing coil FA and tracking coil pair TRa and TRb are located. A rectangular lens holder 220 is formed on the upper surface of the cylinder 210. Lens containers on one side of the upper surface of the lens holder 220 upwardly protrude in which a DVD objective lens La and the CD objective lens Lb are mounted. The lens containers include a first lens container 222 and a second lens container 224. A balance member 226 is mounted on the lens holder 220 opposite to the lens containers 222 and 224 so that the lens holder 220 can be balanced. An axial hole 228 is formed on the weight center of the lens holder 220 in alignment with the center of the cylinder 210. The axial hole 228 is connected to a shaft 232 protruding in parallel with the axial direction from the center of the yoke 230, so that the cylinder 210 is connected to the yoke 230. Protrusions 234 are formed from the yoke 230 between the shaft 232 and the cylinder 210 and in parallel with the axial direction. The inner periphery of the yoke 230 corresponding to the periphery of the cylinder 210 has a pair of magnets 236 in an electromagnetic interaction with the focusing coil FA and a tracking coil pair TRa and TRb.
In the optical pickup actuator as shown in FIG. 4, the lens holder 220 is rotated on the upper surface of the yoke 230, and the lens containers 222 and 224 protrude from the lens holder 220. Accordingly, the height of the optical pickup actuator becomes higher, so that minimization of the optical pickup actuator can not be obtained.
Further, the lens holder 220 has the balance member 226. Accordingly, as the lens holder becomes heavy, the resonance frequency of the lens holder becomes low, so that the optical pickup actuator is influenced by a parasitic resonance.